Digital images have become commonplace. They are displayed on the display screens employed in computer systems and televisions, such as CRTs and LCDs. They are printed as hard copy on media with devices like laser and inkjet printers.
Digital devices can be used to zoom in or out on an object in a digital image. A camera, digital television, a cellular telephone, personal digital assistant, a computer, or other digital device enlarges a selected portion of the original image by creating a new image from that portion. Digital images are comprised of arrays of pixels and the enlarged image is made up of pixels from the selected portion of the original. A pixel is a small, discrete element in the display screen that can be illuminated with a particular color and degree of brightness. The term pixel is also used to refer to the datum that define the color and intensity of the physical pixels in the display device. Each pixel in the selected portion of the original image appears once. In the enlarged image, however, each pixel from the selected portion of the original image appears at least once and may appear more than once.
In practice, it is not uncommon for a viewer to repeatedly scale an image using different scale factors. The scale factor indicates the degree of enlargement. A scale factor of 2× doubles the size of the image. A scale factor of 1.5× enlarges the image by 50 percent, and so on. The viewer may enlarge a selected portion of an image by 2×, but see that it is too big. The viewer may then enlarge the portion by 1.5×, but see that it is not quite big enough. In a trial and error fashion, the viewer may repeatedly scale the selected portion seeking the optimum resolution for viewing.
When repeatedly enlarging a selected portion of an image, it is desirable to maintain the center of the selected portion at the center of each enlarged image. If the central pixels are not so maintained, objects in the successive enlarged images may appear to jitter. A jittering object is an object that appears to move irregularly in small amounts up, down, or sideways. Image jitter annoys and distracts the viewer.
Image jitter is not a problem if the pixels at the center of the selected portion all have the same color and intensity. Jitter is perceptible where there are transitions in color or intensity in the selected portion of the original. For instance, consider a portion of an image of the same color and intensity, such as a patch of clear blue sky. Image jitter is unlikely when repeatedly enlarging such a portion. On the other hand, consider a portion of an image having a black line in the middle of it, such as a border, on a white background. The line may define a portion of text, a border on a dialog box, or the edge of an object. The line may be only one pixel wide. Repeatedly enlarging this portion may result in image jitter. There is no way to know the composition of the pixels of the selected portion in advance. But it is portions with transitions, such as those in the second example, that are typically of interest to the viewer.
When an original image is enlarged repeatedly, the central pixels of the original image may or may not be maintained at the center of the enlarged image. Whether the central pixels of the original image are maintained at the center of the enlarged image depends on the scale factor. For some scale factors, the central pixel in the enlarged image will be the same as the central pixel in the original image. But for other scale factors, the central pixel of the original image will be shifted in the enlarged image. Again, there is no way to know the scale factors the viewer will choose in advance.
Accordingly, there is a need for a method and apparatus for expanding digital image data to create enlarged images for display, the images being enlarged by varying degrees and in a way in which a selected pixel, such as a central pixel, in an original image is maintained at a particular location in each of the enlarged images regardless of the scale factor used.